The invention relates to an apparatus for removal of paint mist from exhaust air in an exhaust system of a spray-painting operation. More particularly, the invention relates to an improved method for washing paint mist from the exhaust air by turbulent mixing with water.
The present invention is an improvement over the type of device disclosed in West German O.S. No. 2,800,668. Generally, this known exhaust system draws exhaust air, by means of reduced pressure, into a mixing chamber, over and/or through a washing liquid, and through a stabilizing chamber whereby paint mist is removed from the exhaust air.
The important features of a paint washing device of the type over which the present invention is an improvement include vigorous turbulent mixing of the exhaust air with the washing liquid whereby the paint is absorbed onto washing liquid droplets and precipitated into a sump of the apparatus to coagulate and form a sludge. The sludge can be removed as required.
The washing liquid used is primarily water. Commercially available chemicals and anti-foamer are normally added to the water to coagulate the paint and cause it to sink for removal from the exhaust system sump.
The mixing chamber is constructed as an open-ended cylinder, usually with a generally circular cross-section. The open ends serve as exhaust outlets from the chamber. The inlet is an exhaust air feed funnel the smaller opening of which terminates approximately tangentially to the wall of the cylinder at a mid-portion of the cylinder.
The exhaust air and the washing liquid entering the cylindrical mixing chamber at the mid-portion thereof flow in a spiral path outwardly toward the open ends of the mixing chamber. To insure better mixing, it is usual to include an elongated flange extending radially inwardly from the inner wall to break up the flow of water and provide turbulent mixing with the exhaust air as they spiral toward the open ends of the chamber. This flange structure normally extends along the full length of the inner wall of the chamber.
It is also known from O.S. No. 2,800,668, to block the inlet of the mixing chamber partially to provide turbulent flow mixing of liquid and exhaust gas as they flow into the mixing chamber.
The present invention is based on the realization that the greater and more vigorous the period of turbulent flow or mixing of liquid and exhaust air, the better will be the washing ability of the device.
One can obtain greater and more vigorous turbulent flow by increasing the speed at which air moves through the mixing chamber. For practical situations, there are certain design characteristics which limit the volume of air flowing through a washing device for a paint mist exhaust system; these include the size of the spray booth to be ventilated. Thus, increasing the flow rate through the exhaust system requires adjusting the size of the washing chamber and, concurrently, adjusting exhaust fans which generate the pressure differential required to increase the velocity of air there through as required. However, the energy consumption of usual exhaust fan systems is exponentially proportional to the differential pressure change (depending on conditions to be maintained). As can readily be understood, more than a moderate increase in differential pressure to be maintained across the system can result in an unacceptably great increase in the use of energy for the system. Thus, a substantial increase in the efficiency of known systems e.g. that shown in German O.S. No. 2,800,668, if it can be accomplished at all, would be expected to require unacceptably high increases in energy use.
It is further noted that presently available systems can barely maintain a cleaning capacity of about 3 mg. paint particles per cubic meter of air leaving the washing system for a normal spray-painting operation. As usual exhaust air volumes of a spray booth system can reach up to 600,000 cubic meters per hour or more, there can be several gallons of paint per hour passing through the duct work and fans of a typical exhaust system. As is readily realized, this can cause quite a mess, as well as the clogging of various portions of the system, including the exhaust fans, thus necessitating closing down the system for clean up, usually bimonthly.
In addition, the exhaust air leaving most washing systems still contains such a high paint particle content that the exhaust air is normally vented into the atmosphere, thus carrying away the still-contained paint and any solvents used in the spray operation. Although the exhaust air can be filtered with dry filters (which remove the paint but not the solvents) and recycled into unmanned zones--or the solvents also can be reclaimed in a condensation (distillation) type recovery unit and the exhaust air then fed back into manned zones--such cleaning operations are impractical because of the rapid clogging of the systems with the paint. As about 50% of the energy consumption in a spray shop is in preparing the air for the spray booths--to heat and condition the air including removal of dust etc.--the ability to recycle such air would produce tremendous energy and cost savings.
Therefore, a washing system which could reduce the amount of paint left in the exhaust air well below the presently considered "good" 3 mg. per cubic meter, would result in considerable energy saving as well as a cleaner operation.